This invention relates to substrates for magnetic disks, and methods for coating and texturing (i.e. roughening) such substrates.
It is known in the art to use glass substrates for magnetic disks because glass is suitably impact resistant. In other words, a glass substrate will not be deformed if a read-write head strikes the disk. However, because glass is hard and brittle, it is difficult to mechanically texture glass substrates, and instead, they are typically textured with an HF chemical etching process prior to being covered with an underlayer and a magnetic film. See, for example, U.S. Pat. No. 5,087,481, issued to Chen, et al. and U.S. Pat. No. 4,833,001, issued to Kijima, et al., each of which is incorporated herein by reference. (The reason that magnetic disk substrates are textured is to reduce stiction between the magnetic disk and a read-write head during use.) Unfortunately, even chemical texturing of glass substrates is difficult and expensive, and it is particularly difficult to obtain consistent results with chemical texturing.
It is also known in the art to use aluminum substrates covered with plated NiP to manufacture magnetic disks. Of importance, aluminum is a light weight material and NiP can be polished and mechanically textured. Further, the NiP is hard and prevents the aluminum substrate from becoming dented if the read-write head strikes the disk. (In contrast, it is unnecessary to cover glass substrates with plated NiP because glass is more impact resistant than NiP, and to the best of the present inventor's knowledge, NiP is not plated onto glass substrates in the manufacture of magnetic disks.) Also of importance, NiP lends itself to being mechanically textured.
NiP-coated aluminum substrates are typically prepared by subjecting an aluminum substrate to a zincate process, during which it is immersed in an alkaline zinc immersion bath. Such a process is described at pages 593 to 595 of "Modern Electroplating", edited by Lowenheim, published by John Wiley & Sons, Inc. in 1974, incorporated herein by reference. The zincate process results in formation of a thin Zn layer on the aluminum substrate. The substrate is then plated with NiP by electroless plating. See, for example, F. Pearlstein, "Electroless Plating", published at pages 710 to 725 of "Modern Electroplating", also incorporated herein by reference. During this process, the Zn layer is consumed and replaced by a NiP layer. The NiP layer is then polished and mechanically textured.
In recent years, industry has started experimenting with laser texturing. During laser texturing, a laser beam is used to form craters in the NiP layer. See, for example, U.S. Pat. Nos. 5,062,021 and 5,108,781, each issued to Ranjan et al., incorporated herein by reference. Also see Baumgart, et al., "Safe Landings: Laser Texturing of High-Density Magnetic Disks", Data Storage, March 1996, also incorporated herein by reference. When using this process, the substrate must be smooth prior to laser texturing.
One advantage of laser texturing is that it can be used to texture a small portion of the disk surface to form a "take-off and landing zone" (also called a "contact-start-stop zone" or "CSS zone") where the read-write head takes off and lands when the disk drive is turned on and off. The remainder of the disk (the "data zone") is used to store data. In contrast, it is difficult to limit texturing to a take-off and landing zone with the above-described HF chemical etching process.
It has previously been difficult to use a laser to texture a magnetic disk with a glass substrate because laser light is not readily absorbed by a glass substrate. One has to use a relatively powerful laser to texture glass. See, for example, Teng, et al., "Laser Zone Texture on Alternative Substrate Disks", published at the 1996 Intermag Conference, incorporated herein by reference. It would be desirable to be able to laser texture a glass substrate instead of the above-mentioned chemical etching techniques.